The present invention relates to a method of press forming of plastic material, and more particularly aims to provide a method of press forming of melt plastic material in which a series of forming operations from material charge to press forming are performed automatically.
Conventionally, press forming of plastic material by a press having open type moulds has been limited to a relatively narrow field, e.g. relatively large formed member which is unsuitable for injection moulding and formed member including long glass fibers which are difficult to form by injection moulding. Usually moderate size members of polypropylene or polyethylene are not formed by open type moulds. Starting material for the open type mould is usually supplied as a plastic sheet from a material supplier. When the sheet is made of stampable thermoplastic material, the sheet is softened by heating and is charged by personnel on a lower mould of the open type press. Such handling of each sheet is the most popular process in an open type mould press. However, plastic sheets are very expensive and reheating of the sheet necessitates much electric power. Further, workability and productivity can not be improved, and safety problems are inherent to the operation.
The conventional handling of the reheated sheet also involves problems that accurate positioning of the sheet is difficult so that non-uniform thickness or shortage tends to occur in the product, and that as the cured plastic sheet is reheated melt patches tend to occur on the surface. To avoid uneven heating the sheet is heated to relatively high temperature to result in surface overheat which tends to deteriorate material properties.